Yuh-ling Lee Chen1,*, Houng-Yung Chen2, Yen-Huei Lin1,2

ABSTRACT: The South China Sea (SCS) is an oligotrophic marginal sea in the tropical-subtropical western North Pacific. Its warm, stratified and nitrate depleted water is an ideal habitat for the nitrogen- (N) fixing cyanobacteria Trichodesmium.
Its abundant presence was not revealed, however, by our results, compiled from 7 cruises conducted between 2000 and 2002 in the northern SCS covering 2 springs (March), 3 summers (June to July) and 2 falls (October). Averaged Trichodesmium density
in the surface water was 77 x 103 trichomes m-3 with great spatial and temporal variations. No definite spatial distribution pattern was recognizable. Its depth- (0 to 60 m) integrated abundance was much lower in the springs (57 to
1360 x 103 trichomes m-2) than either in the summers (480 to 19229 x 103 trichomes m-2) or falls (667 to 21308 x 103 trichomes m-2). This might be attributed to the relatively shallower
nitracline and lower water temperature in the spring than the warm seasons. The year-round abundance was positively related to (p < 0.05) both nitracline depth and surface water temperature. In the warm seasons when the surface temperature ranged
between 27.1 and 31.3°C, only the nitracline depth was significantly related to the Trichodesmium abundance. Nitracline depth that was greater in the Kuroshio water compared to the SCS water also plays a decisive role in controlling the
Trichodesmium abundance in the 2 regions, being greater in the Kuroshio than in the SCS. A sediment trap system deployed near the Luzon Strait in the SCS collected Trichodesmium samples as deep as 374 m. It showed a maximum downward flux of
Trichodesmium at 24.69 x 103 trichomes m-2 d-1 in September and decreased to <1 x 103 trichomes m-2 d-1 from December to March. The sinking rate, when expressed as percentage of
the integrated abundance, was 0.1 to 0.3 and 0.05% for the warm and the cold seasons, respectively. Despite its surface dwelling nature, Trichodesmium showed a downward flux when it was transported from the Kuroshio to the SCS where it became
dissipated.